IEEE section 802.3ab, which is hereby incorporated by reference, specifies physical layer parameters for 1000BaseT (gigabit) communications channels. The gigabit communications channel employs four twisted pairs of cable. Signals transmitted over the cable are degraded by signal attenuation, return loss, echo, and crosstalk.
Referring now to FIG. 1, a gigabit Ethernet communications channel 10 is shown. The communications channel 10 includes two nodes 12 and 14 that transmit and receive one gigabit per second (Gbps). The node 12 includes transceivers 16-1, 16-2, 16-3, and 16-4 and the node 14 includes transceivers 18-1, 18-2, 18-3 and 18-4. Each transceiver transmits at 250 Mbps. The transceivers 16 and 18 are connected to opposite ends of twisted pairs 20-1, 20-2, 20-3, and 20-4. For example, the transceiver 16-1 is connected to one end of the twisted pair 20-1. The transceiver 18-1 is connected to the opposite end of the twisted pair 20-1. Each transceiver 16 and 18 includes a transmitter 24, a receiver 26, and a hybrid circuit 28.
The transmitter 24 of the transceiver 16-1 generates a five level pulse amplitude modulated (PAM-5) signal that is transmitted by the transmitter 24 and the hybrid circuit 28 of the transceiver 16-1 onto the twisted pair 20. The hybrid circuit 28 and the receiver 26 of the transceiver 18-1 receive the PAM-5 signal. The hybrid circuit 28 enables bi-directional transmission over the same twisted pairs by filtering out the transmit signal at the receiver 26.
Attenuation refers to signal loss of the twisted pair between the transmitter of one receiver and the receiver of another transceiver and is caused by several factors including skin effect. To minimize the effect of attenuation, the lowest possible frequency range that supports the required data rate is typically used. Return loss quantifies the amount of power that is reflected due to cable impedance mismatches.
Echo occurs when signals are transmitted and received on the same twisted pair. Echo is caused by residual transmit signals and cable return loss. Crosstalk occurs due to signal coupling between twisted pairs that are in close proximity. For example, the twisted pairs used in 1000BaseT are affected by crosstalk from adjacent twisted pairs. Near end crosstalk (NEXT) is crosstalk at the transmitter end of the twisted pair. Far- and crosstalk (FEXT) is crosstalk at the receiver end of the twisted pair. Crosstalk is preferably minimized to improve receiver symbol recovery.
Referring now to FIG. 2, the transceiver 16 includes a transmitter line driver 50 that receives a transmitter signal 52. The transmitter line driver 50 outputs a multi-level signal to a load such as a matched resistor 54. A transformer 58 couples the transceiver 16 to a twisted pair 60. A replica signal generator 64 outputs a replica of the transmitter signal 52 to a summer 66. A received signal 68 is also input to the summer 66.
Since the communications channel transmits and receives on the same twisted pair 60, the replica transmitted signal is cancelled or subtracted from the received signal 68. In addition, compensation for NEXT and echo is performed. An output of the summer 66 is input to an optional low pass filter (LPF) 70. An output of the LPF 70 is input to an analog to digital converter (ADC) 74. An output of the ADC 74 is input to a summer 78. A linear echo compensation circuit 82 and NEXT compensation circuit 83 (for NEXT12, NEXT13, and NEXT14) are also input to the summer 78. A signal (TAcomp) with NEXT and linear echo compensation is output by the summer 78. Additional details concerning the transceiver 16 can be found in “Active Resistive Summer for a Transformer Hybrid”, U.S. patent application Ser. No. 09/920,240, filed Aug. 1, 2001, and “A Method and Apparatus for Digital Near-End Echo/Near-End Crosstalk Cancellation with Adaptive Correlation”, U.S. patent application Ser. No. 09/465,228, filed Dec. 17, 1999, which are hereby incorporated by reference.
Referring now to FIG. 3, the transmitter line driver 50 is shown further and typically includes a plurality of positive current cells 84 and negative current cells 86. A transmitter driver control 88 selectively switches the positive and negative current cells 84 and 86 on and off to produce positive and negative signal levels. For example, the transmitter line driver for 1000BaseT employs five symbol levels −2, −1, 0, +1, and +2, which are usually implemented as 0V, +/−0.5V and +/−1V. Future communications systems may include additional symbol levels for increased bandwidth. For example, future signal levels may include 0, +/−2, +/−4, +/−6, and +/−8 signal levels.
Referring now to FIG. 4, a conceptual illustration of the transmitter line driver 50 is shown. The positive current cells 84 can be thought of as a plurality of individual current sources 90-1, 90-2, 90-3, . . . , and 90-n that are switched by switches SWP1, SWP2, SWP3, . . . , and SWPn. The negative current cells 86 can be thought of a plurality of individual current sources 92-1, 92-2, 92-3, . . . , and 92-m that are switched by switches SWN1, SWN2, SWN3, . . . , and SWNm. Typically, m=n. Referring now to FIG. 5, an exemplary positive current cell 96 is shown. In FIG. 6, an exemplary negative current cell 98 is shown. As can be appreciated, other positive and negative current cells can be utilized.
When the transmitter line driver 50 is operated in a Class A operating mode, the number of positive current cells that are turned on/off for a transition from a first signal level to a second signal level is equal to the number of negative current cells that are turned off/on. When the transmitter line driver 50 is operated in a Class B operating mode, the number of positive current cells that are turned on/off for a transition from a first signal level to a second signal level is not equal to the number of negative current cells that are turned off/on. The advantage of Class B operation is reduced power consumption as compared with Class A operation.
Referring now to FIG. 7, Class A operation of the positive and negative current cells 84 and 86 for nine symbol levels is shown. As can be appreciated, when switching between signal level 6 and signal level −4, there are an equal number of positive and negative current cells being turned on and off. In particular, five positive current cells are being turned off and five negative current cells are being turned on.
Referring now to FIG. 8, exemplary Class B operation of the positive and negative current cells 84 and 86 is shown. As can be appreciated, when switching between signal level 6 and signal level −4, an unequal number of positive and negative current cells are turned on and off. In particular, six positive current cells are turned off and four negative current cells are turned on. While Class B operation provides reduced power consumption, the asymmetry of Class B operation causes nonlinear echo that degrades performance.